Fracability Evaluation Based on the Three-Dimensional Geological Numerical Simulation of In Situ Stress: Case Study of the Longmaxi Formation in the Weirong Shale Gas Field, Southwestern China

Quantitative evaluation of fracability is essential for hydraulic fracturing design, with the distribution of in situ stress being a key parameter. This study focused on evaluating fracability based on three-dimensional (3D) geological numerical simulation of in situ stress. First, a 3D geological model was established based on seismic data and the geological setting of a field of interest. Thereafter, the anisotropy of the mechanical parameters and boundary conditions was used to calculate the in situ stress on the 3D model. The analytic hierarchy process (AHP) method was then used to calculate the fracability related to the horizontal stress difference coefficient, rock mechanical brittleness, silicon content, cohesion, and internal friction angle. The horizontal stress difference coefficient was obtained by 3D geological numerical simulation, and the remaining parameters were obtained from logging data. The results revealed that the mechanical parameters exhibited significant anisotropy within the field of interest. The in situ stress depended on the structural depth and anisotropy of the mechanical parameters. The fracability index along the well trajectory differed at different well depths. The calculation results were verified using the Meijer G-function. With the parameters used in the AHP method, the in situ stress was determined by a 3D geological numerical simulation, logging data, and laboratory experiments, from which the fracability index was obtained. The results verified by the Meijer G-function indicated that obtaining the fracability index based on the 3D geological numerical simulation of in situ stress was advantageous with respect to complex formations.